(1) Field of the Invention
This invention relates to reinforced cell frames for use in filter press type electrolytic cells.
(2) Description of the Prior Art
The prior art has given considerable attention to the electrode coating materials, the diaphragm or iron exchange membrane composition, and the like. As a result very little attention has been directed to much needed improvements aimed at reducing the cell frame cost and to means and methods for improving the same in the area of filter press type electrolytic cells. In filter press type cells, the frame material is generally thick and of solid construction since the frame is under considerable compressive force when assembled into a multiple unit filter press cell. When the filter press type cell is operated under greater than atmospheric pressure even greater compressive forces are applied to the frame than when the cell is operated under atmospheric pressure.
Electrode sections for filter press type cells are generally employed as large planar surfaces in order to economically use the metal mesh forming the electrode as well as the planar membranes, and because of the very high cost of present filter press cell construction materials such as titanium, ruthenium, nickel, fluorocarbon and carboxylic acid substituted polymer membranes, it is highly desirable to maximize current densities and to reduce voltage coefficients in operating chlor-alkali cells so as to best utilize the mechanical and electrical advantage of the materials of construction employed.
Prior art cell frame construction has used heavy member construction and/or cylindrical shape. Heavy walled construction either with solid, electrolyte resistant metals such as titanium and nickel or steel lined with an electrolyte resistant metal is expensive and consumes large amounts of metal. For these reasons chlor-alkali cells of the filter press type have not been extensively utilized. Generally, filter press type chlor-alkali cells are constructed so that the ion exchange membranes are clamped under pressure between flanges of the filter press frames. Filter press cells are usually of the bipolar type.
Early filter press type ion exchange membrane type cells were constructed of heavy plastic frames. Generally these cells were of the bipolar type which utilized a solid sheet or back plate which functioned as a divider between the cells and was fabricated integrally with the frame. Bipolar cells of this type followed the well developed filter press fabrication principles. The integral frame-back plate construction provided excellent stiffening of the frame structure. The frames for these cells were molded from hard rubber, filled polypropylene, polyester-fiberglass, polyester or any other material that was chemically resistant to the cell environment. Generally, the anode frame was formed of these plastic materials while the cathode frame was formed from steel. The size of filter press type ion-exchange membrane cells tends to be limited in size by the high cost for very large molds and the tendency for warping that tends to occur in heavy plastic frames when the frames are subjected to operating temperatures during actual cell use. The plastic parts employed in these cells tend to have a high coefficient of expansion compared to the electrodes and other metal parts of the cells. Distortion is caused within the cell when the various parts expand or contract at different rates. Filled plastic frames are susceptible to corrosion by chlorine especially when the filler material is a material such as calcium carbonate. In addition, the presence of calcium and magnesium in these plastic frames has been found to be detrimental because of the adverse effect of these elements on ion-exchange membrane life.
Therefore, because of such deficiencies, present day filter press ion exchange membrane cells generally employ metal frames. Generally the metal frames are made of titanium on the side of the frame facing the anode and nickel on the side of the frame facing the cathode. Metal frame construction provides advantages in high strength, small cross sectional area in the structural members, corrosion resistance, resistance to warping and compatibility with respect to coefficient of expansion with metal electrode surfaces. The very high fabrication cost of metal frames has led to attempts to reduce the cost such as by the employment of plastic frames which will give the advantages that metal frames offer without the high cost.
In U.S. Pat. No. 4,402,813, Kircher et al disclose a composite fiber reinforced plastic frame wherein a thin corrosion resistant liner material having a highly reinforced structural core is partially formed by wrapping roving layers of glass fiber impregnated with a catalyzed thermosetting resin having high strength and a low coefficient thermoexpansion.
In U.S. Pat. No. 4,378,286, Eng et al disclose a filter press type electrolytic cell for the electrolysis of brine having a plurality of rectangular frames having U-shaped or channel-shaped walls, the open side of each of which wall members face inwardly. Cross members partially cover the openings of the framing walls to reduce distortion during closing of the cell. In U.S. Pat. No. 4,315,811, Kircher discloses a reinforced metal cell frame having U-shaped channels which provide resistance to compression.